This invention relates generally to a hermetically-sealed radio-frequency (RF) interconnection and method of making said interconnection, and, more particularly, to a hermetically-sealed, RF interconnection, feed-through or connector with an aluminum housing.
Typically, high reliability RF electronics circuits are packaged in hermetically-sealed steel housings to protect the circuits from corrosive environments and humidity. These sealed housings use electrical interconnections that are also hermetic that penetrate the housing and are needed to transfer electrical signals into and out of the housing. Hermetic RF feed-through interconnections are usually incorporated to transfer the high-frequency electrical signals. The RF interconnections generally comprise an electrically conductive center conductor, an outer electrically conductive housing, and an electrically insulating material, such as an insulating glass, hermetically sealed to the center conductor and the outer housing. These interconnections are typically manufactured from weldable stainless steels and high-melting-temperature, silicate-based, insulating glasses that require sealing temperatures above 900.degree. C. RF interconnections installed into the hermetically-sealed housings are also typically made of weldable stainless steels. These hermetic packages are used in aerospace applications, such as communications satellites, microwave communications equipment, and military communications and radar systems which require a hermetic seal to avoid contamination of the RF devices inside. Being made of steel, these packages contribute to the heavy and undesirable weight of the final application assembly.
Electrical connectors and electrical housings made of an aluminum alloy, being of lighter weight, are preferred but have not been used for these hermetic assemblies because heretofore directly sealable and weldable, hermetic, RF interconnections with suitable electrical and chemical-durability properties joined with aluminum bodies could not be made. Previous RF interconnections have generally been made either with steel bodies or made with aluminum bodies with a transition joint made of steel between the insulating glass and the aluminum body. The transition joint is a seal typically made between steel or iron-nickel alloy rings and pins where the transition joint is made by explosively bonding a steel ring to an aluminum ring. A high-melting-temperature glass is used to seal to the inside of the steel ring. This sub-assembly is then welded into an aluminum housing assembly. The use of transition joints requires additional processing steps compared to the traditional manufacture of hermetic steel RF interconnections.
Sharp et al., in U.S. Pat. Nos. 5,041,019 and 5,109,594, issued on Apr. 20, 1991 and May 5, 1992, respectively, describe an electrical connector that utilizes a transition joint where a layer of steel is explosively bonded to a layer of aluminum, thus enabling a connector to be made that has substantially an aluminum-alloy body. These inventions are hermetically sealed transition joints for use with a microwave package. The inventions are seals between steel or iron-nickel alloy rings and pins. These seals are then explosively bonded to an aluminum ring, which is then welded into the aluminum package. Electronic signals are allowed to enter and exit the package via pins contained within the feed-throughs and power connectors. The feed-throughs contain a pin of desired metal surrounded by a bead of glass which is surrounded by a layer of cold rolled steel, stainless steel and/or iron-nickel alloy. This layer is laser-welded to a second layer of an aluminum alloy. The pin serves as an electrical connection to communicate with the electronic circuit inside the package. The glass provides electrical isolation between the pin and the package. Manufacture of these connectors with transition joints requires additional processing steps for making the steel-to-aluminum joint compared to connectors where the glass is directly attached to a steel or aluminum package. Moreover, the designed package is a microwave package and is not designed for RF applications, where the impedance must be closely matched along the entire connector length.
The reliability of the feed-through with a transition joint is typically very poor. Besides the difficulty of a good attachment during manufacture, these joints commonly fail upon thermal cycling. There are two recognized reasons. First, poor nickel and/or gold plating of the packages, feed-throughs and power connectors can result in excessive leaching of the plated metals during soldering, thereby inhibiting soldering. The second reason is mismatched expansion between the aluminum or aluminum alloy of the package and the feed-throughs and power connectors. The coefficient of thermal expansion of aluminum/aluminum alloys is approximately 22.times.10.sup.-6 in/C/in vs that of cold rolled steel and stainless steel at approximately 12-18.times.10.sup.-6 and iron-nickel alloys at approximately 7.times.10.sup.-6. This mismatch in expansion during thermal cycling can create stresses which can result in the loss of the hermeticity and expensive rework and repeat of testing in these devices. In frequent situations upon multiple recurrence, the package becomes useless and is discarded.
Some progress has been made to develop connectors with aluminum bodies. D'Alessandro, in U.S. Pat. No. 3,685,005, issued on Aug. 15, 1972, describes a hermetically sealed connector made with an aluminum body. However, the connector is not designed for RF applications and does not meet the electrical impedance requirements to be able to be used in RF applications.
Viret et al., in U.S. Pat. No. 5,367,125, issued on Nov. 22,1994, describe a hermetic connector consisting of an aluminum shell, a phosphate-based glass seal and a copper/beryllium connecting pin. The connectors described utilize a vitreous or glass material with a required modifying agent for increasing the working temperature range of the glass material. Various embodiments of the invention require pre-oxidation of the aluminum body in a toxic chromic acid bath and nickel-plating and pre-oxidation of the conducting Cu/Be pin. The embodiments are designed for electrical applications.
Some progress has been made in developing a glass that can be heremetically sealed to materials such as aluminum alloys. Brow et al., in U.S. Pat. No. 5,262,364, issued on Nov. 16, 1993, describe a glass composition for hermetically sealing to high-thermal-expansion materials such as aluminum alloys, stainless steels, copper, and copper/beryllium alloys.
The dielectric constant of the insulator material used to make any coaxial RF interconnection is important since the dielectric constant of the glass material, .kappa., technically derived from the electrical relative permittivity of the material, controls the electrical characteristic impedance of the interconnection.
The electrical Characteristic Impedance (Z.sub.0) of a coaxial cylindrical geometry RF interconnection is inversely proportional to the dielectric constant and directly proportional to diameter of the interconnection, as given by the relationship, EQU Z.sup.0 .varies.(1/.sqroot..kappa.) log.sub.10 (D.sup.i /d.sup.o).
where:
Z.sub.0 is the Characteristic Impedance of the geometry; PA1 .kappa. is the dielectric constant of the insulator material; PA1 D.sup.i is the inner diameter of the outer conductor; PA1 d.sup.o is the outer diameter of the inner conductor. PA1 a) mixing amounts of Na.sub.2 O, K.sub.2 O, Al.sub.2 O.sub.3, P.sub.2 O.sub.5, B.sub.2 O.sub.3, and one or more metal oxides selected from the group consisting of BaO, PbO, CaO, and MgO or a mixture thereof; PA1 b) heating the mixture; PA1 c) casting the melt into a mold as a glass rod, cooling the melt, and then annealing the cooled melt into a preformed glass rod; PA1 d) coring the preformed glass rod and cutting the rod to length; PA1 e) preparing an insert by placing a center conductor coaxially into the cored glass rod and sealing the conductor to the glass rod by heating; PA1 f) sealing the insert to an aluminum housing by heating the insert and housing.
Therefore, given a constant center conductor diameter and desired Characteristic Impedance (e.g., 50 ohms), a higher dielectric constant means that the outer connector must have a larger diameter and the overall size and weight of the RF connector will increase. Advantageous are therefore glasses that can be sealed to aluminum alloys and that have low dielectric constants.
To make a directly sealable and weldable, hermetic RF interconnection with aluminum bodies, an insulating glass is required that can be sealed to the aluminum at temperatures below the melting point of aluminum alloys, that have thermal expansion coefficients that can be matched to the electrically conductive center contact material, that can be impedance-matched to make a suitable RF connector, and that has high chemical durability, mechanical strength, and very low gas permeability. The melting point of typical aluminum alloys is about 550.degree. C. compared to that of a conventional silicate glass which has a sealing temperature generally higher than about 1000.degree. C. Furthermore, the thermal expansion coefficient of copper and copper-beryllium alloys preferred for high electrical conductivity pins is generally higher than that of conventional silicate glasses.